Lateral insulated gate bipolar transistor having a retrograde doping profile in base region and method of manufacture thereof

ABSTRACT

In a semiconductor device of the present invention, a first base region  16  is extended to a part under a gate electrode  7  while having a vertical concentration profile of an impurity that increases from the surface of a semiconductor layer  3  and becomes maximum under an emitter region  5 , and the length in the lateral direction from a point where the impurity concentration becomes maximum located under an end of the gate electrode  7  to the boundary with a second base region  15  is not smaller than the length in the vertical direction from the point where the impurity concentration becomes maximum to the boundary with the second base region  15.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/662,680,filed Apr. 10, 2010, which is a divisional of application Ser. No.11/943,614, filed Nov. 21, 2007, now U.S. Pat. No. 7,719,086, issued May18, 2010, which claims the priority of Japanese Patent Application No.2006-332718, filed Dec. 11, 2006.

FIELD OF THE INVENTION

The present invention mainly relates to an insulated gate bipolartransistor (hereafter abbreviated as “IGBT”).

BACKGROUND OF THE INVENTION

With a marked price reduction of color plasma display panels (hereafterabbreviated as “PDP”) in recent years, the cost reduction ofincorporated semiconductor products has been demanded. For a scandrivers IC for color PDPs, an integrated circuit wherein high voltagelateral IGBTs and low voltage circuits are integrated in a chip areoften adopted on a SOI (silicon on insulator) substrate.

In order to reduce the costs of the above-described IC, it is effectiveto reduce the size of the lateral IGBT that occupies a large area in achip. For this reason, the current capability of the lateral IGBT atforward bias must be improved. The current capability of the lateralIGBT is measured on at least on-state voltage and characteristics ofdestroying devices, such as SOA (safe operating area) and ESD(electrostatic discharge).

FIG. 9 shows a general lateral IGBT on a SOI substrate.

An N-type semiconductor layer 3 is bonded on a supporting substrate 1via a buried insulating film 2. On the surface of the N-typesemiconductor layer 3, a P-type base region 4 and an N-type bufferregion 8 are formed. On the surface of the P-type base region 4, anN⁺-type emitter region 5 and a P⁺-type base contact diffusion region 10are diffused. On the surface of the N-type buffer region 8, a P⁺-typecollector region 9 is diffused. On the P-type base region 4, a gateelectrode 7 extends above a field oxide film 11 via a gate insulatingfilm 6. On the N⁺-type emitter region 5 and the P⁺-type base contactdiffusion region 10, an emitter electrode 12 is formed, and on theP⁺-type collector region 9, a collector electrode 13 is formed.

The factors that determine on-state voltage and SOA will be described.

On-state voltage chiefly depends on the resistance component when thedevice is in the on-state. The channel resistance formed below the gateinsulating film 6 is one of major resistance components, and thereduction thereof is important. Whereas, the control of latch up isimportant for SOA.

As shown in FIG. 10, since a parasitic NPN transistor composed of theN⁺-type emitter region 5, the P-type base region 4, and the N-typesemiconductor layer 3 is present, the equivalent circuit of the lateralIGBT has a thyristor structure as shown in FIG. 11. Once this thyristorcauses latch-up, the thyristor cannot be controlled by gate voltage anda low-impedance state is produced bringing in breakdown. The conditionof generating latch up is:α_((NPN))+α_((PNP))≧1where α_((NPN)) and α_((PNP)) are the common base current gains of theparasitic NPN transistor and PNP transistor, respectively. Thereby, tocontrol latch-up, it is also important to lower the current gain a ofboth bipolar transistors.

Since the PNP transistor relates to on-state voltage, the excessivelowering of the current gain α reduces priority to the MOS transistor.Therefore, to prevent latch-up, the action of the parasitic NPNtransistor (turn-on) is suppressed. The common base current gain α ofthe parasitic NPN bipolar transistor is given by the following formula:α=[1−(D _(E) /D _(P))(N _(B) /N _(E))(W/L _(E))]·[1−W ²/2L_(B) ²]where D_(E) is the diffusion coefficient of minority carriers in theemitter region, D_(P) is the diffusion coefficient of minority carriersin the base region, N_(B) is the impurity concentration in the baseregion, N_(E) is the impurity concentration in the emitter region, W isthe width of the base region, L_(E) is the diffusion length of minoritycarriers in the emitter region, and L_(B) is the diffusion length ofminority carriers in the base region. In order to lower α valueobviously, it is effective to increase N_(B), reduce N_(E), and increaseW.

To suppress the turn-on of the parasitic NPN transistor, it is effectiveto reduce the base resistance Rb below the emitter region. When thelateral IGBT is in the on state and the collector current is increased,the voltage drop increases in the base resistance Rb. When the voltagedrop exceeds the built-in potential V_(bi), turn-on is started.Therefore, the reduction of the base resistance Rb is particularlyimportant.

Normally in the device structure of a lateral IGBT, as shown in FIG. 9,a P⁺-type base contact diffusion region 10 of a high concentration isinserted under the N⁺-type emitter region 5. Alternatively, a dedicatedhigh-concentration diffusion layer having larger diffusion depth may bediffused from the surface instead of the P⁺-type base contact diffusionregion 10. In any case of the P⁺-type base contact diffusion region 10and the dedicated high-concentration diffusion layer, since channelresistance is increased when diffusion reaches under the gate electrode7, a sufficient margin is required in the distance L1 to the gateelectrode 7 shown in FIG. 10. The base resistance becomes the serialresistance of the distance L1 of a low concentration and the distance L2of a high concentration.

Therefore, the turn-on of the parasitic NPN transistor depends on thevoltage drop in the distance L1 having a higher resistance. The distanceL1 is easily varied by the widths of the gate electrode 7 and theP⁺-type base contact diffusion region 10, and their misalignment. Inaddition, the lateral spread of diffusion is easily influenced byvariation of heat treatment (drive-in temperature and time) then it'svariation changes the distance L1.

To reduce such variation factors, a device structure wherein ahigh-concentration region is formed in the entire area under the emitterregion has been proposed.

In the invention described in Japanese Patent Application Laid-Open No.10-242456, as shown in FIG. 12, a high-concentration P-type region 14 iscontinuously added under a P-type base region 4. In the inventiondescribed in Japanese Patent Application Laid-Open No. 2002-270844, asshown in FIG. 13, a high-concentration P-type region 14 is formed underan emitter region 5. The high-concentration P-type region 14 is formedby self-aligning to the gate electrode 7 so as not to affect the channelportion.

In both of the above-described conventional techniques, since on-statevoltage depends on the channel resistance of the P-type base region 4,and latch-up depends on the high-concentration P-type region 14.Thereby, the on-state voltage and the latch-up can be independentlycontrolled. Furthermore, since the high-concentration P-type region 14does not reach the channel region, the variation of channel resistancecan be reduced.

DISCLOSURE OF THE INVENTION

Problems in the device structure of conventional techniques will bedescribed below.

As described above, it is desirable to lower the current gain α of theparasitic NPN transistor for preventing latch up. When the state of theparasitic NPN transistor is turn-on, the base potential at the end ofthe emitter region is increased. Then, minority carriers (electrons) arediffused from the end of the emitter region toward the P-type baseregion. Since the high-concentration P-type region is present under theemitter region, current gain lowers. On the other hand, since thelow-concentration P-type base region is present under the gateelectrode, current gain from the emitter region in the lateral directionis higher than current gain in the vertical direction. Consequently,carriers are diffused mainly into the low-concentration region in thelateral direction and arrive at the N-type semiconductor layer.

Therefore, to suppress current gain, the concentration of a P-typeimpurity is increased in the P-type base region under the gate electrodelocated in the lateral direction near the emitter end, andalternatively, the width of the P-type base region is expanded in thelateral direction.

However, to avoid the expansion of the width in the lateral direction ofthe element, the width of the P-type base region under the gateelectrode would rather be reduced as much as possible. For the P-typebase region, all of the above-described conventional techniques havenormal diffusion profiles, specifically Gaussian distribution whereinthe surface concentration is highest. Therefore, if the entire impurityconcentration is increased, the surface concentration is also increased.The increase of the surface concentration not only results in theincrease of channel resistance, but also increases emitter resistancebecause the low concentration of N-type impurity in the emitter regionunder the sidewalls of the gate electrode might be affected. Therefore,the impurity concentration in the P-type base region cannot be set to behigh. Consequently, it is difficult to reduce the width of the P-typebase region under the gate electrode.

What is described above is more important in a high breakdown voltageelement having a breakdown voltage of about 200 V used in a PDP scandriver IC. This is because when a high voltage is supplied between theemitter and the collector, the depletion region spreads toward theP-type base region side, and the base width is narrowed.

In the invention described in Japanese Patent Application Laid-Open No.2002-270844, as shown in FIG. 13, a high-concentration P-type region 14is present only under the emitter region 5, and a low-concentrationP-type region is present under the gate electrode 7.

In the invention described in Japanese Patent Application Laid-Open No.10-242456, as shown in FIG. 12, although a high-concentration P-typeregion 14 extends to under the gate electrode 7, it does not reach thesurface, and a low-concentration P-type base region is present in thelateral direction from the side of the emitter region 5, which is notsufficient.

Furthermore, another problem in the conventional technique will bedescribed.

In the invention described in Japanese Patent Application Laid-Open No.10-242456, since the high-concentration P-type region 14 is presentunder the P-type base region 4, the curvature of the PN junction to theN-type semiconductor layer 3 increases electric fields easily.Therefore, it is not so preferable as a device having high breakdownvoltage of 100 V and above.

In the invention described in Japanese Patent Application Laid-Open No.2002-270844, since the high-concentration P-type region 4 is formed byself-alignment with the gate electrode 7, if the accelerating energy andthe dose of the P-type impurity (boron) are increased, the P-typeimpurity penetrates through the gate electrode 7, and affects thechannel. Therefore, the thickness reduction of the polysilicon gateelectrode is difficult. At present, with a fine process rule, since thepolysilicon film tends to be thinned for relieving steps, and theexisting process cannot be used as it is, there is a disadvantage thatprocess changes are required and lower developing efficiency is lowered.

A semiconductor device according to an embodiment of the presentinvention includes a supporting substrate; a semiconductor layer of afirst conductivity type formed above a main surface of the supportingsubstrate via a buried insulating film; a first base region of a secondconductivity type formed from the surface of the semiconductor layer; asecond base region of the second conductivity type having aconcentration lower than the surface concentration of the first baseregion, and including the first base region fully or partly; a bufferregion of the first conductivity type formed from the surface of thesemiconductor layer apart from the second base region in the lateraldirection; an emitter region of the first conductivity type formed fromthe surface of the first base region; a collector region of the secondconductivity type formed from the surface of the buffer region; a basecontact diffusion region of the second conductivity type formed from thesurface of the first base region or the second base region; a gateinsulating film formed on the second base region at least from an end ofthe emitter region to an end of the second base region; a gate electrodeformed on the gate insulating film; an emitter electrode connected onthe emitter region and the base contact diffusion region; and acollector electrode connected on the collector region; wherein the firstbase region is extended to a part under the gate electrode while havinga vertical concentration profile of an impurity of the secondconductivity type that increases from the surface of the semiconductorlayer and becomes maximum under the emitter region; and the length inthe lateral direction from a point where the impurity concentrationbecomes maximum located under an end of the gate electrode to theboundary with the second base region is not smaller than the length inthe vertical direction from the point where the impurity concentrationbecomes maximum to the boundary with the second base region.

A semiconductor device according to another embodiment of the presentinvention includes a supporting substrate; a second base region composedof a semiconductor layer of a second conductivity type formed above amain surface of the supporting substrate via a buried insulating film; afirst base region of a second conductivity type formed from the surfaceof the second base region; a well region of a first conductivity typeformed in the semiconductor layer apart from the first base region inthe lateral direction; a buffer region of the first conductivity typeadjoining the well region; an emitter region of the first conductivitytype formed from the surface of the first base region; a collectorregion of the second conductivity type formed from the surface of thebuffer region; a base contact diffusion region of the secondconductivity type formed in the first base region; a gate insulatingfilm formed on the second base region at least from an end of theemitter region to an end of the well region; a gate electrode formed onthe gate insulating film; an emitter electrode connected on the emitterregion and the base contact diffusion region; and a collector electrodeconnected on the collector region; wherein the first base region isextended to a part under the gate electrode while having a verticalconcentration profile of an impurity of the second conductivity typethat increases from the surface of the second base region and becomesmaximum under the emitter region; and the length in the lateraldirection from a point where the impurity concentration becomes maximumlocated under an end of the gate electrode to the boundary with thesecond base region is not smaller than the length in the verticaldirection from the point where the impurity concentration becomesmaximum to the boundary with the second base region, and the second baseregion between immediately under the first base region and the buriedinsulating film has a concentration lower than the surface concentrationof the first base region.

A semiconductor device according to another embodiment of the presentinvention includes a semiconductor substrate having two main surfaces; asemiconductor layer of a first conductivity type adjoining to one of themain surfaces of the semiconductor substrate; a first base region of asecond conductivity type formed from the surface of the semiconductorlayer; a second base region of the second conductivity type having aconcentration lower than the surface concentration of the first baseregion, and involving the first base region; a base contact diffusionregion of the second conductivity type formed from the surface of thefirst base region or the second base region; an emitter region of thefirst conductivity type formed from the surface of the first baseregion; a collector region of the second conductivity type adjoining tothe other main surface of the semiconductor substrate; a buffer layer ofthe first conductivity type inserted between the collector region andthe semiconductor layer; a gate insulating film formed at least from anend of the emitter region to an end of the second base region; a gateelectrode formed on the gate insulating film; an emitter electrodeconnected on the emitter region and the base contact diffusion region;and a collector electrode connected on the collector region; wherein thefirst base region is extended to a part under the gate electrode whilehaving a vertical concentration profile of an impurity of the secondconductivity type that increases from the surface of the semiconductorlayer and becomes maximum under the emitter region; and the length inthe lateral direction from a point where the impurity concentrationbecomes maximum located under an end of the gate electrode to theboundary with the second base region is not smaller than the length inthe vertical direction from the point where the impurity concentrationbecomes maximum to the boundary with the second base region.

Furthermore, the base contact diffusion region of the secondconductivity type is extended to under the emitter region.

Furthermore, a method of the manufacture of a semiconductor deviceaccording to the present invention includes the steps of forming a firstbase region of a second conductivity type on the surface of asemiconductor layer; forming a second base region having a lowerconcentration and a larger diffusion depth than the first base region soas to include the first base region fully or partly; forming a bufferregion of the second conductivity type in the semiconductor layer apartfrom the second base region; sequentially forming a gate insulating filmand a gate electrode on the first base region and the second baseregion; forming an emitter region of a first conductivity type on thesurface of the first base region; and forming a collector region of thefirst conductivity type on the surface of the buffer layer; wherein thefirst base region is formed before the step of forming the gateinsulating film using the implantation of boron ions with acceleratingenergy within a range between 150 KeV and 200 KeV, and a dose within arange between 3×10¹³ and 1×10¹⁴ ions/cm².

Furthermore, the first base region and the gate electrode are disposedso that the end portions thereof overlap each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a lateral IGBT, which is a firstembodiment of the present invention;

FIG. 2 is a supplemental sectional view of the lateral IGBT, which isthe first embodiment of the present invention;

FIG. 3 is a graph showing an impurity concentration profile in A-AAcross-section of the lateral IGBT shown in FIG. 2;

FIG. 4 is a graph showing an impurity concentration profile in B-BBcross-section of the lateral IGBT shown in FIG. 2;

FIG. 5 is a sectional view of a lateral IGBT, which is a secondembodiment of the present invention;

FIG. 6 is a sectional view of a vertical IGBT, which is a thirdembodiment of the present invention;

FIG. 7 is a plan view of the lateral IGBT, which is the first embodimentof the present invention;

FIG. 8A is a sectional view of a method of the manufacture of thelateral IGBT, which is the first embodiment of the present invention;

FIG. 8B is a sectional view of a method of the manufacture of thelateral IGBT, which is the first embodiment of the present invention;

FIG. 8C is a sectional view of a method of the manufacture of thelateral IGBT, which is the first embodiment of the present invention;

FIG. 8D is a sectional view of a method of the manufacture of thelateral IGBT, which is the first embodiment of the present invention;

FIG. 9 is a sectional view of a lateral IGBT;

FIG. 10 is a supplemental sectional view of a lateral IGBT;

FIG. 11 is an equivalent circuit schematic of a lateral IGBT;

FIG. 12 is a sectional view of a lateral IGBT of a first conventionalexample;

FIG. 13 is a sectional view of a lateral IGBT of a second conventionalexample; and

FIG. 14 is a graph showing the relationship between on-state breakdownvoltage and dimensional ratio (w/d) of a lateral IGBT of the presentinvention.

DESCRIPTION OF THE EMBODIMENTS

The first base region of an insulated gate bipolar transistor (IGBT) ofthe present invention has the following three characteristics:

First, the first base region has a retrograde profile having the maximum(peak) of the concentration of an impurity of a second conductivity typeunder the emitter region of a first conductivity type, and the impurityconcentration is gradually increased from the surface toward the peak.

Thereby, since the resistance of the base region of the secondconductivity type under the emitter region of the first conductivitytype can be lowered, the effect of suppressing the turn-on of theparasitic NPN transistor can be obtained.

Second, the first base region slightly extends from under the end of thegate electrode toward the collector region of the second conductivitytype. The length in the lateral direction from the maximum point ofimpurity concentration under the end of the gate electrode is notsmaller than the vertical length from the maximum point. Thereby, thequantity of the impurity in the base region of the second conductivitytype under the gate electrode can be not less than the quantity of theimpurity under the emitter region of the first conductivity type, andthe current gain of the parasitic bipolar transistor can be reduced.

Third, the first base region is adjacent to or included in the region ofthe second conductivity type of a low impurity concentration. The P-typeregion of a low impurity concentration is equivalent to the second baseregion or a semiconductor layer.

Thereby, the off-state breakdown voltage can be increased. The firstbase region is formed near the surface, and the diffusion depth is notsignificantly large. Therefore, when the first base region is adjacentto the semiconductor layer of the first conductivity type, due to itscurvature, increasing electric field lowers the off-state breakdownvoltage. According to the present invention, by adjoining the first baseregion with a region of a second conductivity type of a low impurityconcentration that has a relatively large diffusion depth, the reductionof electric field could be obtained, then the off-state breakdownvoltage is improved.

As described above, according to the present invention, there isprovided an excellent device structure that is particularly effectivefor a lateral IGBT of high breakdown voltage, can lower on-state voltageand improve SOA at the same time, and can improve the currentcapability.

The present invention also has effects when applied to a vertical IGBT.In the case of a vertical element, although the collector region and thebuffer region are formed on the back-face side of a substrate, themethod for forming the base region is the same as the method describedfor the lateral IGBT.

As a feature of the manufacturing method, the first base region and thesecond base region having a retrograde profile must be formed beforeforming a polysilicon gate. Thereby, the first base region and thesecond base region can be formed under the gate electrode. Into thefirst base region, boron ions are implanted by a relatively highaccelerating energy within a range between 150 and 200 KeV. Thereafter,heat treatment called drive-in is performed to form a retrograde profilehaving a peak at a depth of about 0.5 μm from the surface.

The present invention will be described referring to each embodiment.

First Embodiment

FIG. 1 shows a sectional view of a lateral IGBT according to the firstembodiment of the present invention; and FIG. 7 shows a plan viewthereof.

In FIG. 1, on the surface of an N-type semiconductor layer 3 of a lowimpurity concentration formed on a supporting substrate 1 via a buriedinsulating film 2, a second P⁻-type base region 15 and an N-type bufferregion 8 are diffused apart from each other. The second P⁻-type baseregion 15 is often shared with a P-type well region of a low voltageNMOS transistor or a P-type offset region of a high voltage PMOStransistor integrated in the same chip, and the surface impurityconcentration is as relatively low as about 1×10¹⁶ cm⁻².

In the second P⁻-type base region 15, a first P-type base region 16 isformed; and further in the first P-type base region 16, an N⁺-typeemitter region 5 and a P⁺-type base contact diffusion region 10 arediffused.

A gate electrode 7 is formed above the first P-type base region 16 andthe second P⁻-type base region 15 via a gate insulating film 6, andextends above a field oxide film 11. On the N⁺-type emitter region 5 andthe P⁺-type base contact diffusion region 10, an emitter electrode 12 isformed. On the N-type buffer region 8, a P⁺-type collector region 9 isformed, and a collector electrode 13 is further formed on the P⁺-typecollector region 9. The first P-type base region 16 is formed under theN⁺-type emitter region 5.

Referring to a plan view of FIG. 7, the P⁺-type collector region 9 has ashape having two corner portions and two straight portions; and theP⁺-type collector region 9 is surrounded by an N-type buffer layer 8 andan N-type semiconductor layer 3. The N⁺-type emitter regions 5 areformed on the straight portions, and are not formed on the cornerportions. Thereby, current crowding the corner portions is suppressed.

FIG. 2 is a supplemental sectional view; and FIG. 3 is a graph showingan impurity concentration profile in the vertical direction in A-AAcross-section shown in FIG. 2. In FIG. 3, the distance from the maximumpoint of impurity concentrations to the boundary point with the secondbase region 15 is represented by “d”. FIG. 4 shows an impurityconcentration profile in the lateral direction in B-BB cross-sectionshown in FIG. 2. The B-BB cross-section is a profile from under the gateelectrode 7 toward the collector region 9 side in the peak of the A-AAcross-section. The boundary between the first P-type base region 16 andthe second P⁻-type base region 15 is the place where the gradient ofimpurity concentration is changed, and the distance from under the endof the gate electrode 7 is represented by “w”.

The PN junction depth of the N⁺-type emitter region 5 and the firstP-type base region 16 is about 0.1 μm. The peak of the concentration ofthe P-type impurity is about 1×10¹⁸ cm⁻³, and is located at a depth of0.5 μm from the surface and under the N⁺-type emitter region 5. Theboundary between the first P-type base region 16 and the second P⁻-typebase region 15 is located at about 1.4 μm from the surface where theconcentration gradient is changed. The concentration of the P-typeimpurity lowers relatively slowly, and the surface concentration becomesabout 2×10¹⁷ cm⁻³. Such a retrograde doping profile can be easily formedby ion implantation of boron at a dose of 6.5×10¹³ ions/cm² and anaccelerating energy of about 180 KeV, and appropriate heat treatment ofdiffusion called drive-in. The surface concentration of the secondP⁻-type base region 15 is about 1×10¹⁶ cm⁻³.

The first P-type base region 16 is extended to increase theconcentration of the P-type impurity in the second P⁻-type base region15 under the gate electrode 7 located in the lateral direction from theend of an emitter region 5.

This is because if the quantity of the impurity in the lateral directionfrom the end of an emitter region 5 is almost the same as the quantityunder the emitter region 5, current gain in the lateral directionlowers, and increase in the current gain in the entire parasitic NPNtransistor is suppressed.

FIG. 14 shows the relationship between on-state breakdown voltage,on-state voltage and “w/d”. We consider on-state breakdown voltagerather than current. On the other hand, “w/d” on the abscissa is setbecause it could indirectly show the ratio of the quantities of theimpurity in the vertical direction and the lateral direction.

If “w/d” is smaller than 1, the on-state breakdown voltage sharplylowers. Especially for on-state breakdown voltage of more than 200 Vwith sufficient margin, “w/d≧1” is required.

When a high concentration P-type region is formed by self-aligning tothe thick polysilicon gate electrode as in the conventional techniques,since the P-type diffusion region is laterally spread below the gateelectrode, but the lateral spread of diffusion is normally shorter thanthe depth of diffusion, “w/d” is steadily smaller than 1. Althoughon-state voltage lowers as “w/d” is smaller, slight saturation is foundwhen “w/d<1”.

Therefore, even if “w/d” is at least 1, since on-state breakdown voltageis sharply lowered, the instability is caused. Since on-state voltage isincreased when “w/d” is excessively large, it is preferable to setwithin the range of “1≦w/d≦2”.

In the present invention, since the P-type diffusion region is formedbefore the polysilicon gate electrode, the P-type diffusion region canbe extended in an optimal length under the gate electrode, and “1≦w/d≦”can be easily realized. Considering the on-state voltage and SOA in thepresent invention, the values of d and w are substantially equal at 0.9μm, and are set as “d/w=1”.

In the first embodiment, although the first P-type base region 16 isformed under the base contact diffusion region 10, and the secondP⁻-type base region 15 is formed under the first P-type base region 16,the first P-type base region 16 may be not formed under the base contactdiffusion region 10, if the base contact diffusion region 10 is formedfrom the surface of the second P⁻-type base region 15.

Second Embodiment

FIG. 5 shows a lateral IGBT according to the second embodiment of thepresent invention, wherein only the following aspects are different fromthe first embodiment.

The lateral IGBT shown in FIG. 5 has a device structure using an SOIsubstrate on which a P-type semiconductor layer 17 is formed, and theP-type semiconductor layer 17 reaches a buried insulating film 2. Into afirst P-type base region 16 as a first base region, an N⁺-type emitterregion 5 and a P⁺-type base contact diffusion region 10 are diffused.

In the same manner as in the first embodiment, the first P-type baseregion 16 is extended to a part under a gate electrode 7 while having avertical concentration profile of a P-type impurity that increases fromthe surface of the P-type semiconductor layer 17 and becomes highestunder the N⁺-type emitter region 5. The lateral length w from themaximum point of the impurity concentration located under an end portionof the gate electrode 7 is not smaller than vertical length d from theabove-described maximum point, and the P-type semiconductor layer 17from immediately under the first P-type base region 16 to the buriedinsulating film 2 has a concentration at least ten times lower than thesurface concentration of the first P-type base region 4.

The impurity concentration of the P-type semiconductor layer 17 is3×10⁺¹⁵ cm⁻³, and is suppressed to at least ten times lower than thesurface concentration of the first P-type base region 16 of 2×10⁺¹⁷cm⁻³. Thereby, when a reverse bias is supplied to the N-type well region18 and the P-type semiconductor layer 17, a depletion region can beextended to the P-type semiconductor layer 17, and the electric field atthe end of the first P-type base region 16 could be reduced.

Third Embodiment

FIG. 6 shows a sectional view of a vertical IGBT according to the thirdembodiment of the present invention.

Because of a vertical element, a P-type collector region 9 is formed onthe back face of a substrate 20, and an N-type buffer region 8 is formedon the P-type collector region 9. An N-type semiconductor layer 3 isformed thereon. Second P⁻-type base regions 15 are spaced between eachother near the surface, and on the surface of the second P⁻-type baseregion 15, a first P-type base region 16 is formed.

Furthermore, an N⁺-type emitter region 5 and a P⁺-type base contactdiffusion region 10 are formed on the surface of the first P-type baseregion 16, and an emitter electrode 12 is formed thereon. A gateelectrode 7 is formed from an end of the N⁺-type emitter region 5 viathe gate insulating film 6. The first P-type base region 16 has aretrograde profile having the peak under the N⁺-type emitter region 5,and is extended under the gate electrode.

In the same manner as in the first embodiment, the first P-type baseregion 16 is extended to a part under the gate electrode 7 while havinga vertical concentration profile of a P-type impurity that increasesfrom the surface of the N-type semiconductor layer 3 and becomes highestunder the emitter region 5. The lateral length w from the maximum pointof the impurity concentration located under an end portion of the gateelectrode 7 is not smaller than the vertical length d from theabove-described maximum point.

In the third embodiment, although the first P-type base region 16 isformed under the base contact diffusion region 10, and the secondP⁻-type base region 15 is formed under the first P-type base region 16,a configuration wherein the first P-type base region 16 is not formedunder the base contact diffusion region 10, can also be formed, and inthis case, the base contact diffusion region 10 is formed from thesurface of the second P⁻-type base region 15 as the second base region.

Fourth Embodiment

FIG. 8 shows a method of the manufacture of an IGBT according to thefirst embodiment of the present invention.

As shown in FIG. 8A, a second P⁻-type base region 15 and an N-typebuffer region 8 are formed on the surface of an N-type semiconductorlayer 3 using ion implantation and heat treatment called drive-in.Thereafter, as shown in FIG. 8B, a LOCOS film 11 is formed, and a firstP-type base region 16 is formed on the surface of the second P⁻-typebase region 15 using boron ion implantation. The accelerating energy is,for example, 180 KeV, and the dose is 6.5×10¹³ ions/cm². As shown inFIG. 8C, after forming a gate insulating film 6, a gate electrode 7 ofpolysilicon is formed so that the end portion thereof overlaps the firstP-type base region 16.

As shown in FIG. 8D, an N⁺-type emitter region 5, a P⁺-type collectorregion 9, and a P⁺-type base contact diffusion region 10 are formed byion implantation. Furthermore, an emitter electrode 12 and a collectorelectrode 13 are formed.

In the fourth embodiment, the P⁺-type base contact diffusion region 10may be extended to under the emitter region 5 to further lower theresistance of the base region. Thereby, the turn-on of the parasitic NPNtransistor can be further suppressed.

The first P-type base region 16 can be formed before the formation ofthe gate insulating film 6 by implanting boron ions using acceleratingenergy within a range between 150 KeV and 200 KeV and a dose within arange between 3×10¹³ and 1×10¹⁴ ions/cm².

In the above-described embodiments, although the case wherein the firstconductivity type is N-type and the second conductivity type is P-typeis described as an example, the first conductivity type can also beP-type, and the second conductivity type can also be N-type.

1. A method of manufacturing a semiconductor device, comprising: forminga first base region of a second conductivity type on a surface of asemiconductor layer; forming below said first base region a second baseregion having a lower concentration and a larger depth of an impurity ofthe second conductivity type than said first base region; forming abuffer region of the second conductivity type in said semiconductorlayer apart from said second base region; sequentially forming a gateinsulating film and a gate electrode on said first base region; formingan emitter region of a first conductivity type on a surface of saidfirst base region; and forming a collector region of the firstconductivity type on a surface of said buffer layer wherein the endportions of said first base region and said gate electrode overlap eachother and a length in a lateral direction from a point where an impurityconcentration of the second conductivity type in said first base regionbecomes maximum under said emitter region and at an end of said gateelectrode to a boundary between said first base region and saidsemiconductor layer is not smaller than a length in the verticaldirection from said point to the underside of said first base region. 2.The method of claim 1, wherein said first base region is formed beforeforming said gate insulating film using the implantation of boron ionswith accelerating energy within a range between 150 KeV and 200 KeV, anda dose within a range between 3×10¹³ and 1×10¹⁴ ions/cm².
 3. The methodof claim 1, wherein said semiconductor layer is formed on a supportingsubstrate via an insulating layer.
 4. The method of claim 1, whereinsaid first base region is formed after forming said second base region.5. The method of claim 1, wherein said semiconductor layer is asubstrate of the first conductivity type.
 6. The method of claim 1,wherein said semiconductor layer is a supporting substrate of the secondconductivity type; said second base region is formed above a mainsurface of said supporting substrate; and said buffer region is formedin a well region of the first conductivity type, spaced internally apartfrom said first base region.
 7. The method of claim 1, wherein accordingto a plan view of said collector region, said collector region has acorner portion and a straight portion and is surrounded by said gateelectrode, and said emitter region is located at the periphery of saidcollector region and is formed corresponding to said straight portion.